The inventive concept is a result of research projects supported by Korea Institute of Energy Technology Evaluation and Planning (Project ID. 2014010031880) of the Ministry of Trade, Industry and Energy.
Compared to other fuel cells, a polymer electrolyte membrane fuel cell (PEMFC) has low operating temperature, high efficiency, high current density, high power density, short starting time, and a rapid response speed in response to a load change. In the PEMFC, a catalyst is respectively applied to an anode and a cathode of a polymer electrolyte membrane (catalyst coated membrane, CCM). Also, the PEMFC includes a membrane electrode assembly (MEA) obtained by attaching a gas diffusion layer onto the catalyst and bipolar plates attached to opposite surfaces of the MEA. The gas diffusion layer (GDL) can be obtained by coating carbon slurry for the microporous layer (MPL) on a carbon substrate including a porous carbon material such as a carbon cloth, carbon felt, and carbon paper.
One of the current issues in the field of hydrogen fuel cell vehicles (HFCVs) is the development of components and materials capable of reducing the volume of existing the components, reducing the component prices, increasing durability thereof, and obtaining fuel cells with high performance. A metal bipolar plate used as a fuel cell bipolar plate in order to decrease the price of a fuel cell vehicle (FCV) and to increase the energy density has a greater stacking pressure than that of a carbon bipolar plate, and accordingly, a pressure per unit area applied to the MEA and the GDL greatly increases and the GDL can collapse. This can cause degradation of the performance and durability of the fuel cell.
In addition, many vehicle manufacturers consider switching the priority of production from recreation vehicles (RVs) to sedans or compact cars. To do this, it is necessary to reduce the volume of fuel cell stacks and also reduce a thickness of the metal bipolar plate and a thickness of the GDL.
Referring to FIG. 1, a metal bipolar plate a according to the prior art has channel formed therein and provides fuel and oxygen to a GDL via the channels. However, a porous bipolar plate b that has been recently developed and applied to an air electrode distributes air, generates air turbulence due to its three-dimensional structure, and provides the air to an entire GDL. Regarding the bipolar plate of the prior art, the GDL and the bipolar plate are in surface-contact with each other. However, in the case of the porous bipolar plate, the GDL and the porous bipolar plate are in point or line-contact with each other. Thus, a pressure applied to the GDL when stacking the porous bipolar plate is much higher than that in the case of the metal bipolar plate having grooves. Therefore, a GDL applied to vehicles needs to have high mechanical strength and reduced thickness.
In addition, stacks of the PEMFC shrinks or expands due to temperature variation or reaction conditions when the PEMFC is driven. As such, pressures applied to components in each stack vary, and a gasket and the GDL act as a spring for buffering the pressure. Most GDLs recover a little bit or rarely recover after being deformed by the pressure. However, since the gasket does not function as a spring by itself, the GDL is expected to have the mechanical characteristics of a spring.
Most of the GDLs of a carbon paper type are made by papermaking carbon fibers in a wet-laid felt process, and thus, generally have an isotropic structure including carbon fibers that are irregularly arranged. However, a GDL with such an isotropic structure has the same structure at any surface, and thus, when a force is applied to a surface directly contacting the GDL in the bipolar plate structure with grooves, the carbon fibers can be broken and compressed. Thus, the GDL can be intruded into the grooves or flow fields in the bipolar plate, that is, intrusions occur. Accordingly, a space through which air or fuel moves is reduced. Such an intrusion causes a pressure drop in the flow fields in the bipolar plate and also increase in the power consumption of a pump. Also, the collapse of the GDL structure due to the carbon fibers broken at the interface between the GDL and the flow fields causes degradation of the durability of the GDL and also reducing a reactive gas or the generated water transfer.